1. Field of the Invention
The present invention relates to a wet skin-pass rolling method for rolling steel sheets.
2. Description of the Related Art
Hitherto, control of hardness of steel sheets, particularly steel sheets to be used in the production of tin plates, is effected by controlling the composition of the material steel in the steelmaking process or by controlling temperature and time in the annealing process. Thus, no attempt has been made to control the steel sheet hardness during skin-pass rolling. Conventionally, skin-pass rolling is conducted in a dry state with the reduction ratio controlled to a constant value which is usually not greater than 1.5%. Such dry skin-pass rolling is conducted for various purposes such as elimination of yield elongation, control of roughness of the steel sheet surface, leveling of the steel sheet and so forth.
In recent years, it has been proposed to conduct skin-pass rolling in wet condition in order to improve productivity and to simplify the process, while reducing production cost. By this method, it is easy to widely vary reduction ratio to control the hardness of product. In this case, the rolling is usually conducted at a rolling reduction of about 3 to 15%.
In order to control the hardness of a steel sheet product by wet skin-pass rolling, it is necessary not only to control the hardness of the mother steel sheet but also to keep the reduction ratio constant.
However, it is difficult to directly control the reduction ratio because of the presence of variations in the thickness of the mother steel sheet. It is, therefore, a common measure to control the reduction ratio by a method which maintains an elongation percentage constant which is computed on the basis of the steel sheet velocities at the entry and delivery sides of the rolling mill. This constant-elongation control method is disclosed, for example, in Japanese Patent Laid-Open No. 62-13209.
The above-mentioned constant-elongation method is based upon the following relationship which always exists between elongation .epsilon. and reduction ratio .gamma. due to the fact that the mass-flow of the material is always constant. EQU .epsilon.=.gamma./(1-.gamma.)
The constant-elongation method mentioned above, however, cannot precisely control the thickness of the rolled sheet, although the hardness can be controlled reasonably well.
Namely, any lack of precision in the thickness of the mother steel sheet formed during cold rolling cannot be corrected by the constant-elongation control method alone. Thus the final product sheet will exhibit a similar lack of precision in the thickness with the result that the quality of the product is seriously impaired. Conversely, a sheet thickness control alone cannot enable a hardness control although the precision of the thickness can be improved.